Life cycle
For the timeframe of the regenerative process, see regeneration cycle. A regeneration cycle or life cycle referred to the number of regenerations a Time Lord normally possessed. Time Lords normally had twelve regenerations, which resulted in thirteen incarnations before permanent death. As such, while the Time Lords possessed the ability to "cheat death", as the Ninth Doctor put it, they were not truly immortal, which the Fifth Doctor believed impossible even for them. In certain cases, however, the Time Lords proved able to bestow additional regenerations. It was believed that a Time Lord could only regenerate twelve times because their symbiotic nuclei could only divide so much. The Valeyard, however, claimed this was imposed by Rassilon to prevent a thirteenth regeneration and keep Time Lords from immortality. As noted by the Tenth Doctor, a Time Lord could die prematurely should they be killed before regeneration could occur. As well as that, weaponised inhibitors were developed with the purpose of preventing regeneration. Curiously, the Eleventh Doctor, who had already expended all twelve of his regenerations, once claimed to Clyde Langer that he could regenerate 507 times. However, he was being flippant and admitted to Clara Oswald that he was on his final life. End of the cycle Without access to additional regeneration energy, a Time Lord would ultimately perish at the end of their thirteenth incarnation. In his thirteenth and final body, Azmael deliberately regenerated past his limit, killing him and Mestor, who had been attempting to possess Azmael's body after his own was destroyed. As witnessed by the Sixth Doctor and Peri Brown, an elderly Time Lord at the end of his final life disintegrated as he "degenerated" into molecules. Beyond thirteen lives Despite the limit of thirteen lives, the High Council of Gallifrey possessed the ability to bestow additional regenerations upon individual Time Lords. While the Master was offered a complete life cycle by Borusa, the Eleventh Doctor was granted an indeterminate amount of additional regenerations through regeneration energy provided from Gallifrey. Without this option, certain Time Lords would seek to extend their lives through other means. Having used all his lives, the Master took to possessing the bodies of non-Gallifreyans. In theory, Time Lords could also be robbed of their regenerations by others of their kind, as the Master attempted to do to the Eighth Doctor via the Eye of Harmony. In a story related by the Valeyard, the thirteenth and final incarnation of the Doctor supposedly composed a set of scrolls which detailed his work to bypass the limit of twelve regenerations. The Valeyard's claim, however, was doubted by the Sixth Doctor. Indeed, the Doctor found what appeared to be his final incarnation on Etarho to have been the Valeyard masquerading as him. Individual cases The Master The Master had expended his initial life cycle, but cheated death by possessing the body of the Trakenite Tremas. In return for retrieving the Doctor from the Death Zone on Gallifrey, Lord President Borusa offered to bestow upon the Master a new life cycle. Ultimately, the Master did not receive the life cycle from the High Council. Eventually, however, the Master, having been resurrected by the Time Lords to fight in the Last Great Time War, possessed the ability to regenerate again. Like the Doctor, it is unclear how many regenerations the Master was given in his new cycle. He regenerated three times and expected to be able to regenerate at least once more. The Doctor Another Time Lord who lived beyond their initial life cycle was the Doctor. However, there was confusion as to how many regenerations he used in his first cycle. Having abandoned their title for the vast majority of their actual ninth incarnation, and wasting a regeneration to prevent changing their current form after their eleventh regeneration, the so-called Eleventh Doctor confirmed he had used all of his regenerations and was in his final incarnation. On the verge of dying of old age during the Siege of Trenzalore, the Doctor's companion Clara Oswald pleaded with the Time Lords through a crack in the universe to change the future and save the Doctor. In response, the Time Lords gave the Doctor a new life cycle by sending him regeneration energy through the crack. This caused an explosive "reset" which restored the Doctor to his youthful form and destroyed the attacking Dalek forces but still required the Doctor to regenerate into his next incarnation. However, the Twelfth Doctor remained uncertain of the number of regenerations he possessed, with even Rassilon not being sure how many were granted to him and also the Master unsure as well. Proto-Time Lords While possessing the ability to regenerate, the life cycles of the Proto-Time Lords created by the Kovarian Chapter were uncertain, with separate individuals possessing different numbers of regenerations. The original, River Song, regenerated twice before sacrificing the remainder of her regeneration energy to save the Eleventh Doctor from death. Lake regenerated twice. Following his first regeneration, Lake, unaware of the limit of his regenerative ability, proceeded to test it by inflicting fatal injuries against his fellow Proto-Time Lords while convincing others to commit suicide. To that end, he was responsible for the deaths of Rindle, Wadi, Creek, Beck, Stream, all of whom were determined to possess a different number of regenerations. Rindle regenerated four times before their remaining regenerations were expended by Lake to power his ship. Ironically, Lake's last victim was his third and final incarnation, a girl who became known as Lily having lost her memory of her prior lives. Having witnessed Lake regenerate into Lily after mortally wounding him in anger for murdering her, River Song was forced to send Lily back in time so she would meet her fate. Tarn had regenerated at least ten times. Brooke, one of the second batch of Proto-Time Lords, was observed to regenerate at least once.